Cold-work die steel and die

ABSTRACT

The present invention relates to a cold-work die steel, comprising by mass %: 0.5 to 0.7% of C; 0.5 to 2.0% of Si; 0.1 to 2.0% of Mn; 5 to 7% of Cr; 0.01 to 1.0% of Al; 0.003 to 0.025% of N; 0.25 to 1% of Cu; 0.25 to 1% of Ni; 0.5 to 3% of Mo; 2% or less (including 0%) of W; and 0.1% or less (excluding 0%) of S, with a remainder being iron and an unavoidable impurity; wherein the following requirements (1) to (3) are satisfied: (1) [Cr]×[C]≦4; (2) [Al]/[N]: 1 to 30; and (3) [Mo]+0.5×[W]: 0.5 to 3.00%, wherein the bracket means a content (%) of an element written therein.

TECHNICAL FIELD

The present invention relates to a cold-work die steel and a die, andmore specifically to a die steel useful as a material of dies used incarrying out cold/worm press forming (stamping, bending, drawing,trimming, etc.) of steel plates for cars, steel sheets for home electricappliances and so on.

BACKGROUND ART

With increases in strength of steel plates and sheets, dies used incarrying out forming of steel plates for cars, steel sheets for homeelectric appliances and the like are required to undergo furtherimprovement in their life. As to the steel plates for car in particular,in consideration of environmental issues and with the intention ofenhancing fuel economy of cars, demand for high-tensile steel plateshaving tensile strengths of about 590 MPa or more has grown sharply.With the increase in demand for such high-tensile steel plates, therehas arisen a problem of damaging the surface coating films of dies at anearly stage and thereby causing “galling” (a seizing-up phenomenonoccurring under press forming) to result in extreme loss of die life.

A die is generally made by giving hard coating treatment to the surfaceof a base material of the die (die steel). In manufacturing the diesteel as a base material, processes of heat treatment or annealing, cutworking and quenching-tempering treatment are generally carried out inorder of mention. In the present specification, there may be cases wherethe quenching treatment and the tempering treatment in particular arereferred to as solution treatment and aging treatment, respectively.

As the die steel (cold-work die steel), not only high-C, high-Cr alloytool steel, which is represented by JIS SKD11, but also high-speed toolsteel having further improved abrasion resistance, which is representedby JIS SKH51, has generally been used so far. Improvements in hardnessof these tool steels are mainly made by precipitation hardening of Crcarbide or Mo, W and V carbides. In addition, low-alloy high-speed toolsteels (usually referred to as matrix high speed steels) which areimproved in both toughness and abrasion resistance by reducing thecontents of alloy elements in JIS SKH51, such as C, Mo, W and V, arecurrently in use as die steels.

A variety of methods aiming further improvements in properties of diesteels have been proposed (e.g. Patent Documents 1 and 2).

Patent Document 1 discloses the cold-work die steel to which properamounts of Ni and Al are added and further Cu is added in an amountappropriate to the amounts of Ni and Al added for the purposes ofreducing the quantities of dimensional changes (changes in dimension) byquenching-tempering treatment, particularly changes in dimension byexpansion under tempering, and increasing the hardness. In thisdocument, it is also described that galling resistance can be improvedby making adjustments to contents of C and Cr and finely dispersing thedistribution of carbide in the texture.

With the intention of attaining properties (hardness and toughness) onthe same levels as those of conventional matrix high speed steels evenwhen quenching is performed at temperatures lower than those adopted forthe conventional matrix high speed steels, Patent Document 2 disclosesthe alloy tool steel that has a microstructure in which Cr-basedM₂₃C₆-type carbide is formed in an amount of 2 to 5 vol % under temperedconditions (conditions before heat treatment), and besides, that has aquenched-tempered microstructure including either V-based MC-typecarbide or Mo- and W-based M₆C-type carbide precipitated in a dispersedstate.

Patent Document 1: JP-A-2006-169624

Patent Document 2: JP-A-2004-169177

DISCLOSURE OF THE INVENTION

As described above, a die is generally made by giving hard coatingtreatment to the surface of a die steel. Examples of general hardcoating treatment currently in use include TD treatment by which a VCcoating film is formed through thermal diffusion, CVD treatment by whichTiC is mainly formed, and PVD treatment by which TiN is mainly formed.Herein, the term “TD treatment” refers to the treatment that C in asteel material is allowed to react with V by immersing the steelmaterial in a bath of fused salt including V and the VC coating filmhaving a thickness of about 5 to about 15 μm is diffused and permeatedon the surface of a base material under high temperature conditions ofabout 900 to about 1,030° C. These hard coating treatments are adoptedas appropriate according to the circumstances of die users and pressmakers. Therefore, it is required to develop die steels havingsatisfactory adaptability to any of the hard coating treatments (orcapable of forming long-life hard coating films). In addition, as amatter of course, die steels are also required to ensure excellent basicproperties (including hardness and toughness).

The invention has been made in view of these circumstances, and objectsthereof are to provide a cold-work die steel which exhibits excellentbasic properties (including hardness and toughness), and besides, whichis adaptable satisfactorily to a variety of hard coating treatments, andto provide a die.

Namely, the present invention provides a cold-work die steel, comprisingby mass %:

0.5 to 0.7% of C;

0.5 to 2.0% of Si;

0.1 to 2.0% of Mn;

5 to 7% of Cr;

0.01 to 1.0% of Al;

0.003 to 0.025% of N;

0.25 to 1% of Cu;

0.25 to 1% of Ni;

0.5 to 3% of Mo and 2% or less (including 0%) of W; and

0.1% or less (excluding 0%) of S,

with a remainder being iron and an unavoidable impurity; and

wherein the following requirements (1) to (3) are satisfied:

[Cr]×[C]≦4;  (1)

[Al]/[N]: 1 to 30; and  (2)

[Mo]+0.5×[W]: 0.5 to 3.00%,  (3)

wherein the bracket means a content (%) of an element written therein.

In addition, the cold-work die steel preferably comprises at least oneof the following (a) to (c):

(a) V in a content of 0.5% or less (excluding 0%);

(b) at least one element selected from the group consisting of Ti, Zr,Hf, Ta and Nb in a total content of 0.5% or less (excluding 0%); and

(c) Co in a content of 10% or less (excluding 0%).

The die of the invention is obtained by using any of the cold-work diesteels specified above.

Because in the cold-work die steels of the invention, as specifiedabove, the alloy components and balances between the specified elementsare appropriately adjusted, they can have high hardness and toughness,and besides, long-life hard coating films can be formed on the surfacethereof even by a variety of hard coating treatments. Dies obtained byusing the cold-work die steels of the invention are particularlysuitable as dies for forming high-tensile steel plates having tensilestrength of about 590 MPa or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1( a) is an optical photomicrograph showing a state in whichgalling occurs at the die surface having a TiN coating film formed byPVD treatment in the case of using JIS SKD11 as a die steel, FIG. 1( b)is an optical photomicrograph of the base material for a die which isprovided with no TiN coating, and FIGS. 1( c) and 1(d) are partiallyenlarged optical photomicrographs shown in FIG. 1( a).

FIG. 2 is a schematic diagram showing a shape of the Charpy impact testpiece used in Examples.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have made extensive studies in order to provide acold-work die steel that can satisfactorily exhibit its basicproperties, such as hardness and toughness, and that has sufficientadaptability to a variety of hard coating treatments. As a result, ithas been found that prevention of exfoliation of a TiN coating filmformed thereon and improvements in hardness and toughness can beachieved by not only controlling the contents of various alloy elementsto within individually specified ranges but also adjusting specifiedelements to have respectively appropriate balances as shown in the aboveitems (1) to (3). As the result of such a finding, it has further beenfound that, even when hard coating treatments of various types,including TD treatment, CVD treatment and PVD treatment, are carriedout, long-life hard coating films can be formed on the surface, therebyachieving the invention.

Details about the achievement of the invention are described below.

First of all, the present inventors have researched causes of impairmentof a TiN coating film formed by PVD treatment given to a die usingconventional JIS SKD11 or matrix high speed steel and the gallingbrought about thereby.

FIG. 1( a) is an optical photomicrograph showing a state in whichgalling occurs at the surface of a die made by using JIS SKD11 as a diesteel and forming a TiN coating film thereon by PVD treatment. Inaddition, an optical photomicrograph of the base material for a diewhich is provided with no TiN coating is shown in FIG. 1( b). The areaslooking whitish in FIG. 1( b) indicate the presence of Cr carbide. FIGS.1( c) and 1(d) are partially enlarged optical photomicrographs shown inFIG. 1( a). As is apparent from FIGS. 1( c) and 1(d), it isunderstandable that hard, coarse Cr carbide (carbide mainly containingCr and Fe and having a size of about 1 μm to about 50 μm) precipitatesout on the surface of the areas in which TiN coating film is exfoliated,and cracks are produced from the carbide spots.

From these observation results, the present inventors find that, sincethe galling occurrence in the TiN coating film begins at spots wherecoarse Cr carbide is present, minimization of the carbide formationallows prevention of exfoliation of the TiN coating film and improvementin die life.

For inhibiting the formation of coarse Cr carbide and thereby increasinglifetime of a TiN coating film formed by PVD treatment, it isappropriate to reduce both contents of C and Cr in a steel. However, atoo large reduction in content of C makes it difficult to form a VCcoating film or TiC coating film with a sufficient thickness by TDtreatment or CVD treatment on the surface of a die steel (basematerial). Therefore, one of the features of the invention is attainmentof the sufficiently thick VC coating film and TiC coating film withoutprecipitating coarse Cr carbide by appropriate control of contents of Cand Cr in a die steel and the product of these contents (the foregoingrequirement (1)).

In the die steel of the invention, formation of the coarse Cr carbide isinhibited in order to increase the lifetime of a TiN coating film formedby PVD treatment. However, unless the Cr carbide is formed, grain growthduring quenching cannot be prevented, and excellent toughness cannot beachieved after quenching. Therefore, another feature of the invention isformation of fine AlN by precise control of content of Al, content of Nand balance between them (the requirement (2)), thereby attainingexcellent toughness after quenching. In the present specification, theexpression “excellent toughness” means that the Charpy impact valuedetermined by the method described in the following Examples section is20 J or more. Additionally, the expression “fine AlN” means AlN havingan average grain size of about 5 μm or less.

In addition, since the die steel of the invention contains fine AlN, thedie steel of the invention thought to have improvements in adhesion tonitride coating films (e.g. CrN and TiN) formed by PVD treatment.

In the die steel of the invention, as mentioned above, both contents ofC and Cr are reduced to low values compared with those in JIS SKD11 as aconventional steel for the purpose of inhibiting formation of coarse Crcarbide. In the invention, therefore, hardness reduction by reduction ofboth contents of C and Cr is supplemented with positive addition ofalloy components (particularly Al, Cu, Ni, Mo and W). More specifically,in the die steel of the invention, high hardness is achievedparticularly through the use of hardening by precipitation of an Al—Niintermetallic compound under the control according to the requirement(2) and secondary hardening by formation of carbide from C and Mo or Wunder the control according to the requirement (3). Additionally, theexpression “high hardness” in the present specification means that themaximum hardness determined by the method described in the followingExamples section is 650 HV or more.

Chemical components in the steel of the invention are described below indetail on an element basis. Additionally, all percentages in the presentspecification are by mass unless otherwise noted. And all percentagesand so on defined by mass are identical with those defined by weight,respectively.

C: 0.5 to 0.7%

C is an element that ensures hardness and abrasion resistance andcontributes to inhibition of HAZ softening. In addition, when a coatingfilm of carbide, such as VC and TiC formed by a TD method or by a CVDmethod, is formed on the surface of a base material for dies, a lowcontent of C therein causes a problem that the coating film formed has asmall thickness, and so on. Considering these circumstances, the lowerlimit of the content of C is set to 0.5% for the purpose of achievingthe above effect effectively. And the content of C is preferably 0.55%or more. However, an excessive content of C causes production of coarseCr carbide and makes it easy for a TiN coating film formed by PVDtreatment to exfoliate. In addition, an excessive content of C causes anincrease in residual austenite content, as a result, the desiredhardness cannot be attained unless aging treatment is performed at ahigh temperature, and besides, a great dimensional change occurs throughexpansion after the aging treatment. Moreover, an excessive content of Caffects adversely the toughness. Therefore, the upper limit of thecontent of C is set to 0.7%. And the content of C is preferably 0.65% orless.

Si: 0.5 to 2.0%

Si is useful as a deoxidizing element at the time of steelmaking, and itis an element that contributes to a hardness improvement and ensuresmachinability. In addition, Si is useful for inhibiting the softening ofmartensite in a matrix by tempering and inhibiting HAZ softening. Forthe purpose of fulfilling such functions effectively, the lower limit ofthe content of Si is set to 0.5%. However, an excessive content of Sibrings about a reduction in toughness. In addition, increases insegregation and dimensional change after heat treatment are caused.Therefore, the upper limit of the content of Si is set to 2.0%. Thecontent of Si is preferably 1.0% or more, more preferably 1.2% or more,and preferably 1.85% or less.

Mn: 0.1 to 2.0%

Mn is an element useful for ensuring hardenability during quenching.However, an excessive content thereof brings about an increase inresidual austenite content, as a result, the desired hardness cannot beattained unless aging treatment is performed at a high temperature, andbesides, the toughness is lowered. Considering these circumstances, thecontent range of Mn is defined as the above. The content of Mn ispreferably 0.15% or more, and preferably 1% or less, more preferably0.5% or less, further more preferably 0.35% or less.

Cr: 5 to 7%

Cr is an element useful for ensuring the proper hardness. Specifically,a too low content of Cr brings about insufficient hardenability duringquenching and leads to partial production of bentonite, as a result, thehardness is lowered, and the abrasion resistance cannot be attained.Moreover, Cr is an element useful for ensuring corrosion resistance ofdies. Therefore, the lower limit of the content of Cr is set to 5%. Andthe content of Cr is preferably 5.5% or more. However, an excessivecontent of Cr causes an increased production of coarse Cr carbide andmakes it easier for a TiN coating film formed by PVD treatment toexfoliate. In addition, an excessive content thereof causes a reductionin durability of the hard coating film through shrinkage after heattreatment. Moreover, an excessive content of Cr affects adversely thetoughness. Therefore, the upper limit of the content of Cr is set to 7%.And the content of Cr is preferably 6.5% or less.

Al: 0.01 to 1.0%

Al is an element useful as a deoxidizer, and besides, it is an elementthat contributes to not only hardness improvement by precipitationhardening of an Al—Ni intermetallic compound, such as Ni₃Al, but alsoinhibition of HAZ softening. Moreover, Al is an important element forattainment of excellent toughness by formation of AlN precipitates inconjunction with N and prevention of grain growth during quenching.Considering these circumstances, the lower limit of content of Al is setto 0.01%. The content of Al is preferably 0.02% or more, more preferably0.03% or more.

In the field of tool steels, for the purpose of improving the quality ofinclusions, the content of Al is generally minimized. In the invention,however, Al is positively added for the purpose of increasing thehardness of a die steel, preferably for the purposes of inhibiting HAZsoftening and preventing grain growth. The positive addition of Al inthe invention makes one of significant differences as compared with therelated arts.

On the other hand, an excessive content of Al brings about a reductionin toughness, and besides, it causes great segregation which leads to anincrease in dimensional change after heat treatment. Therefore, theupper limit of the content of Al is set to 1.0%. And the content of Alis preferably 0.8% or less.

N: 0.003 to 0.025%

N is an important element for attainment of excellent toughness byformation of AlN precipitates in conjunction with Al and prevention ofgrain growth during quenching. For the purpose of attaining excellenttoughness, the lower limit of the content of N is set to 0.003%.However, an excessive content thereof brings about a reduction intoughness. Therefore, the upper limit of the content of N is set to0.025%. And it is preferable that the content of N is 0.004% or more and0.020% or less.

Cu: 0.25 to 1%

Cu is an element necessary to aim at hardness improvement byprecipitation hardening of ε-Cu, and contributes also to inhibition ofHAZ softening. However, an excessive content thereof causes a reductionin toughness, and it tends to produce forging cracks. Therefore, theupper limit of the content of Cu is set to 1%. And it is preferable thatthe content of Cu is 0.30% or more and 0.8% or less.

Ni: 0.25 to 1%

Ni is an element necessary to aim at hardness improvement byprecipitation hardening of an Al—Ni intermetallic compound, such asNi₃Al, and contributes also to inhibition of HAZ softening. In addition,the use of Ni in combination with Cu allows control of hot embrittlementby Cu addition in an excessive amount, and thereby the forging crackscan also be prevented. However, an excessive content thereof causes anincrease in residual austenite content, as a result, the proper hardnesscannot be attained unless aging is performed at a high temperature, andbesides, expansion occurs after heat treatment. In addition, anexcessive content of Ni causes a reduction in toughness. Consideringthese circumstances, the content of Ni is specified to fall within therange specified above. And it is preferable that the content of Ni is0.30% or more and 0.8% or less.

Mo: 0.5 to 3%, and W: 2% or Less (Including 0%)

Mo and W are elements that contribute to precipitation hardening becauseeach of Mo and W forms M₆C-type carbide, and besides, a Ni₃Mointermetallic compound is formed. However, excessive contents of theseelements result in overproduction of those carbides and so on, whichleads to not only a reduction in toughness but also an increase indimensional change after heat treatment. Therefore, the content of Moand the content of W are specified so as to fall in the above-specifiedranges, respectively. In the invention, Mo is an essential element,while W is an optional element. However, they may be used incombination. The suitable lower limit of the content of W is 0.02%. Andit is preferable that the content of Mo is 0.7% or more and 2.5% orless, and that the content of W is 0.05% or more and 1.5% or less.

S: 0.1% or Less (Excluding 0%)

S is an element useful for ensuring machinability. From the viewpoint ofensuring machinability, it is recommended that the content of S be0.002% or more, preferably 0.004% or more. However, an excessive contentthereof results in occurrence of welding cracks. Therefore, the upperlimit of the content of S is set to 0.1%. The content of S is preferably0.07% or less, more preferably 0.05% or less, further more preferably0.025% or less.

Further, it is necessary for the die steel of the invention to fulfillthe following requirements (1) to (3) (wherein the bracket means thecontent (%) of each element written therein).

(1) [Cr]×[C]≦4

The requirement (1) is set for the purpose of inhibiting the productionof coarse Cr carbide. When the product of [Cr]×[C] is more than 4,coarse Cr carbide is formed to result in easy exfoliation of TiN coatingfilms. In addition, when this product is more than 4, there occurs notonly degradation in durability of hard coating films but also increasein dimensional change after heat treatment. Thus, [Cr]×[C] is preferably3.8 or less, more preferably 3.7 or less. From the viewpoints ofreducing the formation of coarse Cr carbide, inhibiting the dimensionalchange after heat treatment and the like, the smaller the lower limit ofthis product is, the better it is. However, further consideringsignificant achievement of the effects from the addition of Cr and C, itis preferably basically 0.8.

(2) [Al]/[N]: 1 to 30

The requirement (2) is set for the purpose of forming fine AlN andensuring toughness after quenching. When [Al]/[N] is too small or toolarge, it becomes difficult to form fine AlN precipitates, so excellenttoughness cannot be achieved. Therefore, it is important that thebalance between them is precisely controlled. And [Al]/[N] is preferably2 or more and 20 or less.

(3) [Mo]+0.5×[W]: 0.5 to 3.00%

Mo and W, as mentioned above, are elements that contribute toprecipitation hardening, and the requirement (3) is selected as aparameter for mainly ensuring hardness improvement by precipitationhardening of these elements. In addition, the control of this parameterallows satisfactory inhibition of HAZ softening. In order to achievethese effects effectively, the lower limit of the requirement (3) is setto 0.5%. However, excessive contents of Mo and W result in addition ofexcess carbides, which leads to not only a reduction in toughness butalso an increase in dimensional change after heat treatment. Therefore,the upper limit of the requirement (3) is set to 3.00%. The lower limitof the requirement (3) is preferably 1.0%, more preferably 1.2%, and theupper limit of the requirement (3) is preferably 2.8%. In therequirement (3), (0.5), the coefficient of [W], is defined byconsidering that the molecular weight of Mo is about ½ in comparisonwith that of W.

The basic components in the steel of the invention are as mentionedabove, with the remainder being iron and unavoidable impurities. Theunavoidable impurities are elements unavoidably mixed e.g. in theprocess of manufacturing, with examples including P and O. In general,the content of P is preferably 0.05% or less, more preferably 0.03% orless, and the content of O is preferably 0.005% or less, more preferably0.003% or less, further more preferably 0.002% or less. In addition, forthe purpose of improving other properties, the following optionalcomponents may further be included.

V: 0.5% or Less (Excluding 0%)

V contributes to an improvement in hardness by forming carbide such asVC, and besides, it is an element effective in inhibiting HAZ softening.In addition, when a diffusion hardening layer is formed by givingnitriding treatment, such as gas nitriding, salt bath nitriding orplasma nitriding, to the surface of a base material, it is an effectiveelement for improvement in surface hardness and increase in hardeninglayer depth. For achieving these effects effectively, it is appropriatethat V be basically added in an amount of 0.05% or more. However, Vadded in an excessive amount lessens the amount of C dissolved in solidand causes a reduction in hardness of the martensite texture as amatrix, and besides, it reduces the toughness. Therefore, when V isadded, the upper limit of the content thereof is set to 0.5%. Thecontent of V is preferably 0.4% or less, more preferably 0.3% or less.

At Least One Element Selected from the Group Consisting of Ti, Zr, Hf,Ta and Nb: 0.5% or Less in Total (Excluding 0%)

All of these elements are nitride-forming elements, and they contributeto a finely dispersed state of their nitrides and AlN, accordingly theyare elements allowing prevention of grain growth and contribution toimprovement of toughness. For achievement of such effects effectively,it is basically appropriate that 0.01% or more of Ti, 0.02% or more ofZr, 0.04% or more of Hf, 0.04% or more of Ta and 0.02% or more of Nb becontained. However, when the contents of these elements become too high,the amount of C dissolved in solid is lessened to result in a hardnessreduction of martensite. Therefore, it is preferable that the totalcontent of these elements is set to 0.5%. The total content of theseelements is preferably 0.4% or less, more preferably 0.3% or less.Additionally, these elements may be contained alone or in combinationwith two or more thereof.

Co: 10% or Less (Excluding 0%)

Co is an element effective in heightening an Ms point and reducingresidual austenite, and thereby enhancing the hardness. For achievementof such an effect effectively, it is basically appropriate that thecontent of Co be 1% or more. However, an excessive content thereofbrings about rises in cost and so on. Therefore, it is appropriate thatthe upper limit of content of Co be set to 10%. The content of Co ispreferably 5.5% or less.

The invention further relates to dies obtained by using the die steelsdescribed above. In the invention, though there is no particularrestriction as to the manufacturing method of the dies, a manufacturingmethod which can be adopted is e.g. as follows: After producing theabove steel by melting, the steel is subjected to hot forging, and thensoftened by undergoing heat treatment or annealing (e.g. by being keptat about 700° C. for 7 hours, and then subjected to furnace cooling toabout 400° C. at an average cooling rate of about 17° C./hour, andfurther to standing to cool). Thereafter, the resultant is crude-workedinto intended forms by e.g. a cutting work, and then hardened so as toacquire an intended hardness by undergoing solution treatment attemperatures ranging from about 950° C. to about 1,150° C., andsubsequently undergoing aging treatment at temperatures ranging fromabout 400° C. to about 530° C.

EXAMPLES

Now, the invention will be illustrated in more detail by reference tothe following examples, but the invention should not be construed asbeing restricted by these examples. In carrying out the invention, it ispossible as a matter of course to make changes and modifications asappropriate so long as they conform to the foregoing and followingimports, and any of modes undergoing such changes and modifications areincluded in the technical scope of the invention.

A variety of steel species listed in Table 1 were used and, from each ofthese steel species, 150 kg of ingot was produced by melting in a vacuuminduction melting furnace. Then, each ingot was heated to a temperaturein a range of about 900° C. to about 1,150° C., and forged into twoplates each having a size of 40 mmT×75 mmW×about 2,000 mmL. Thereafter,each plate obtained was slowly cooled at an average cooling rate ofabout 60° C./hour. After cooling to a temperature of 100° C. or less,the resultant was re-heated up to a temperature of about 850° C., andsubsequently cooled slowly at an average cooling rate of about 50°C./hour (heat treatment or annealing).

The following tests (1) to (3) were carried out on each of the materialsthus heat-treated or annealed.

(1) Hardness Test (Determination of Maximum Hardness)

Test species basically having a size of 20 mmT×20 mmW×15 mmL were cutfrom each of the heat-treated or annealed materials, and used as thetest specimens for hardness measurement. Each test specimen wassubjected to the following heat treatment.

Solution treatment (quenching treatment): heating at temperaturesranging from about 1,020° C. to about 1,030° C. for 120 minutes→aircooling→aging treatment (tempering treatment): keeping for about 3 hoursat a temperature in a range of about 400° C. to about 560° C.→standingto cool

While changing the tempering temperature within the range of about 400°C. to about 560° C. range as described above, hardness measurements weremade with a Vickers hardness tester (manufactured by ΛKΛSHI Co., Ltd.,ΛVK standard, load of 5 kg), and the maximum hardness (HV) wasdetermined. In these examples, test specimens showing maximum hardnessof 650 HV or more in the measurements were regarded as acceptable. Thetest results are shown in Table 2.

(2) Toughness Test (Measurement of Charpy Impact Value)

Each of the heat-treated or annealed materials was subjected to thefollowing heat treatment.

Solution treatment (quenching treatment): heating at temperaturesranging from about 1,020° C. to about 1,030° C. for 120 minutes→aircooling→aging treatment (tempering treatment): keeping for about 3 hoursat temperatures ranging from about 400° C. to about 560° C.→air coolingor standing to cool

Then, test species each having a V-notch section of 10-mmR as shown inFIG. 2 were cut therefrom, and used as test specimens for toughnessmeasurement (Charpy Impact test specimens). Charpy impact testing wascarried out on these specimens, from which absorption energy at roomtemperature (Charpy impact value) was determined. Three test specieswere taken for each Charpy impact testing, and the average thereof wastaken as Charpy impact value. When the Charpy impact value obtained inthis testing was 20 J or more, the test specimen exhibiting such aCharpy impact value was regarded as excellent in toughness. The resultsobtained are shown in Table 2.

(3) Property Evaluation of Hard Coating Film

(3-1) Formation of Hard Coating Film

Test pieces basically having a size of 4 mmt×φ50 mm were cut from eachof the heat-treated or annealed materials, subjected to the same heattreatment as in the toughness test, and used as test specimens forproperty evaluation of hard coating films. On separate surfaces of thesetest specimens, a VC coating film, a TiC coating film and a TiN coatingfilm were formed by TD treatment, CVD treatment and PVD treatment,respectively, under general conditions.

(3-2) Thickness Measurement of Hard Coating Film

Photographs of each of the hard coating films formed in the foregoingmanner (VC, TiC and TiN coating films) were taken under a magnificationof 2,000 by use of a scanning electron microscope (SEM), and thicknessmeasurements at 5 points randomly chosen from them were carried out. Theaverage of the values measured at the 5 points was taken as thethickness (μm) of each hard coating film. In these examples, testspecimens allowing formation of both a VC coating film and a TiC coatingfilm having a thickness of 7.0 μm or more were regarded as acceptable.The results obtained are shown in Table 2.

(3-3) Measurement of Exfoliation Limit Load of Hard Coating Film

An exfoliation limit load was measured on each of the hard coating films(VC, TiC and TiN) by pin-on-disk testing. Specifically, a diamondindenter having a tip of 200-μm radius (R) was indented into and made totravel through each hard coating film under conditions that the loadincreasing rate was 100 N/min and the indenter travel rate was 10mm/min, and the load (N) applied to the point where the hard coatingfilm was exfoliated primarily was evaluated as the exfoliation limitload. In these examples, test specimens ensuring an exfoliation limitload of 20 N or more on any of the hard coating films were regarded asacceptable. The results obtained are shown in Table 2.

TABLE 1 [Mo] + [Cr] × [Al]/ 0.5 × No. C Si Mn Cr Al N Cu Ni Mo W S P O VOthers [C] [N] [W] 1 1.49 0.35 0.42 12.10 0.050 0.0130 0.05 0.08 1.040.35 0.005 0.018 0.0015 0.25 — 18.03 3.85 1.22 2 1.01 1.06 0.60 8.380.330 0.0068 0.40 0.44 0.91 0.39 0.007 0.019 0.0007 0.09 Nb: 0.1 8.4648.53 1.11 3 0.25 1.32 0.28 4.95 1.091 0.0148 3.01 2.95 1.20 0.02 0.0040.018 0.0013 0.20 — 1.24 73.72 1.21 4 0.40 1.35 0.25 4.45 1.030 0.01403.00 2.98 1.21 0.02 0.004 0.019 0.0013 0.20 — 1.78 73.57 1.22 5 0.601.00 0.40 5.87 0.009 0.0170 0.04 0.67 0.93 0.02 0.004 0.020 0.0015 0.32— 3.52 0.53 0.94 6 0.58 1.01 0.42 5.95 0.017 0.0154 0.30 0.29 0.95 0.020.004 0.018 0.0014 0.28 — 3.45 1.10 0.96 7 0.58 1.01 0.42 5.95 0.0500.0165 0.30 0.29 0.95 0.02 0.004 0.018 0.0014 0.28 — 3.45 3.03 0.96 80.59 1.02 0.43 5.95 0.100 0.0165 0.30 0.30 0.96 0.02 0.004 0.019 0.00150.29 — 3.51 6.06 0.97 9 0.58 1.01 0.42 5.97 0.220 0.0164 0.29 0.29 0.950.02 0.005 0.018 0.0014 0.28 — 3.46 13.41 0.96 10 0.60 1.02 0.42 5.970.310 0.0165 0.30 0.30 0.95 0.02 0.004 0.018 0.0015 0.28 — 3.58 18.790.96 11 0.58 1.01 0.43 5.95 0.300 0.0195 0.30 0.29 0.95 0.02 0.004 0.0190.0014 0.28 — 3.45 15.38 0.96 12 0.59 1.02 0.44 5.96 0.550 0.0196 0.280.30 0.96 0.02 0.005 0.018 0.0014 0.29 — 3.52 28.06 0.97 13 0.58 1.020.43 5.97 1.050 0.0194 0.30 0.29 0.95 0.02 0.005 0.018 0.0014 0.28 —3.46 54.12 0.96 14 0.58 1.75 0.42 5.95 0.100 0.0165 0.29 0.30 0.95 0.020.004 0.018 0.0013 0.28 — 3.45 6.06 0.96 15 0.58 1.02 1.10 5.96 0.1100.0161 0.30 0.30 0.95 0.02 0.004 0.019 0.0014 0.29 — 3.46 6.83 0.96 160.60 1.02 0.42 5.95 0.100 0.0165 0.73 0.75 0.95 0.02 0.004 0.018 0.00140.28 — 3.57 6.06 0.96 17 0.58 1.02 0.42 5.95 0.100 0.0165 0.30 0.30 1.700.02 0.004 0.019 0.0014 — — 3.45 6.06 1.71 18 0.60 1.01 0.43 5.96 0.1100.0162 0.29 0.30 0.95 0.02 0.080 0.018 0.0014 0.28 — 3.58 6.79 0.96 190.59 1.02 0.42 5.95 0.110 0.0165 0.30 0.30 0.96 0.02 0.004 0.018 0.00130.28 Ti: 0.04 3.51 6.67 0.97 20 0.58 1.02 0.43 5.95 0.100 0.0165 0.290.29 0.95 0.02 0.004 0.018 0.0014 0.29 Nb: 0.1 3.45 6.06 0.96 21 0.601.01 0.42 5.96 0.110 0.0162 0.30 0.30 0.96 0.02 0.004 0.019 0.0015 0.28Zr: 0.1 3.58 6.79 0.97 22 0.58 1.01 0.44 5.95 0.100 0.0163 0.29 0.300.95 0.02 0.005 0.019 0.0014 0.29 Hf: 0.1, 3.45 6.13 0.96 Ta: 0.1 230.59 1.02 0.43 5.97 0.100 0.0165 0.30 0.29 0.95 0.02 0.004 0.018 0.00130.28 Co: 5.2 3.52 6.06 0.96 24 0.58 2.21 0.42 5.96 0.110 0.0165 0.290.30 0.96 0.02 0.004 0.018 0.0014 0.28 — 3.46 6.67 0.97 25 0.60 1.022.10 5.97 0.100 0.0165 0.30 0.30 0.96 0.02 0.005 0.018 0.0013 0.29 —3.58 6.06 0.97 26 0.58 1.02 0.42 5.96 0.100 0.0164 1.50 1.48 0.95 0.020.004 0.018 0.0014 0.28 — 3.46 6.10 0.96 27 0.59 1.02 0.43 5.95 0.1100.0165 0.30 0.29 0.20 0.19 0.004 0.019 0.0014 0.29 — 3.51 6.67 0.30 280.60 1.01 0.42 5.95 0.100 0.0162 0.30 0.30 2.98 0.05 0.005 0.019 0.00130.28 — 3.57 6.17 3.01 29 0.59 1.02 0.43 5.96 0.100 0.0164 0.30 0.29 0.960.02 0.004 0.018 0.0015 0.60 — 3.52 6.10 0.97 30 0.60 1.01 0.42 5.950.110 0.0165 0.29 0.30 0.95 0.02 0.005 0.018 0.0014 0.28 Ti: 0.25, 3.576.67 0.96 Nb: 0.3 31 0.58 1.02 0.43 5.97 0.100 0.0260 0.30 0.29 0.960.02 0.005 0.019 0.0013 0.29 — 3.46 3.85 0.97 Unit: mass %, Remainder:iron and unavoidable impurities

TABLE 2 Charpy Maximum Impact Thickness of Hard Exfoliation Limit Loadof Hardness Value Coating Film (μm) Hard Coating Film (N) No. HV J VC-TDTiC-CVD TiN-PVD VC-TD TiC-CVD TiN-PVD 1 690 10 7.8 6.7 5.1 27 22 17 2720 13 7.5 7.9 5.0 23 23 18 3 685 22 4.3 4.3 5.1 12 10 33 4 710 17 6.56.2 4.9 18 15 32 5 700 15 7.5 7.3 5.0 24 25 31 6 710 30 7.4 7.5 5.0 2726 32 7 715 35 7.3 7.5 5.0 25 26 30 8 724 35 7.5 7.6 5.0 26 24 29 9 72835 7.4 7.7 5.1 25 27 30 10 729 35 7.3 7.8 5.0 27 25 33 11 734 31 7.5 7.54.9 24 23 31 12 750 25 7.5 7.5 5.0 25 24 29 13 745 18 7.4 7.4 4.9 28 2830 14 726 21 7.3 7.7 5.0 23 24 32 15 719 22 7.5 7.6 5.0 26 23 30 16 72123 7.9 7.5 5.1 25 26 30 17 730 22 7.4 7.5 5.0 20 21 31 18 722 15 7.8 7.55.0 28 29 30 19 709 40 7.3 7.3 5.1 21 22 33 20 711 41 7.5 7.6 5.0 22 2130 21 708 39 7.5 7.4 4.9 20 22 31 22 710 28 7.6 7.8 5.0 20 22 32 23 72435 7.5 7.3 4.9 24 24 30 24 729 17 7.3 7.7 5.1 25 26 33 25 730 16 7.9 7.55.0 26 25 28 26 728 17 7.5 7.6 5.1 24 23 31 27 648 20 7.4 7.5 5.0 29 3030 28 741 17 7.3 7.6 5.1 21 22 32 29 718 15 7.6 7.8 5.0 28 29 30 30 60020 7.7 7.9 5.1 24 25 31 31 717 16 7.5 7.5 5.1 24 24 30

As is apparent from Tables 1 and 2, each of the steels Nos. 6 to 12 and14 to 23, which fulfills all the requirements of the invention, isexcellent in all of the (maximum) hardness, toughness (Charpy impactvalue), thickness of the VC or TiC coating film and exfoliation limitload of the hard coating film (the VC coating film, TiC coating film orTiN coating film). By contrast, the steels Nos. 1 to 5, 13 and 24 to 31,each of which does not fulfill at least one of the requirements of theinvention, have the following problems.

The steels Nos. 1 and 2 are insufficient in exfoliation limit load ofthe TiN coating film because all of their contents of C, contents of Crand [Cr]×[C] values are too high, and because they contain coarse Crcarbide. In addition, there are reductions in their toughness becausethey are too high in contents of C and Cr.

The steels Nos. 3 and 4 are insufficient in thicknesses of the VC andTiC coating films because of their low contents of C, and as a result,the exfoliation limit load of those coating films are reduced.

The steel No. 5 is insufficient in toughness because of its low contentof Al and small value of [Al]/[N].

The steel No. 13 is insufficient in toughness because of its highcontent of Al and large value of [Al]/[N].

All the steel Nos. 24, 25 and 26 are insufficient in toughness becauseit is too high in content of Si in the steel No. 24, it is too high incontent of Mn in the steel No. 25 and it is too high in contents of Cuand Ni in the steel No. 26.

The steel No. 27 is insufficient in hardness because of its low contentof Mo and small value of [Mo]+0.5×[W].

The steel No. 28 is insufficient in toughness because of its large valueof [Mo]+0.5×[W].

The steel No. 29 is insufficient in toughness because it contains V asan optional element in the excessive amount.

The steel No. 30 is insufficient in hardness as a result of the contentof C dissolved in solid being reduced by addition of Ti and Nb asoptional elements in the total amount in excess of 0.5%.

The steel No. 31 is insufficient in toughness because of its too highcontent of N.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2007-294326filed on Nov. 13, 2007, and their contents are incorporated herein byreference. In addition, all the references cited herein are incorporatedas a whole.

INDUSTRIAL APPLICABILITY

Because in the cold-work die steels of the invention, as specifiedabove, the alloy components and balances between the specified elementsare appropriately adjusted, they can have high hardness and toughness,and besides, long-life hard coating films can be formed on the surfacethereof even by a variety of hard coating treatments. Dies obtained byusing the cold-work die steels of the invention are particularlysuitable as dies for forming high-tensile steel plates having tensilestrength of about 590 MPa or more.

1. A cold-work die steel, comprising by mass %: 0.5 to 0.7% of C; 0.5 to2.0% of Si; 0.1 to 2.0% of Mn; 5 to 7% of Cr; 0.01 to 1.0% of Al; 0.003to 0.025% of N; 0.25 to 1% of Cu; 0.25 to 1% of Ni; 0.5 to 3% of Mo and2% or less (including 0%) of W; and 0.1% or less (excluding 0%) of S,with a remainder being iron and an unavoidable impurity; wherein thefollowing requirements (1) to (3) are satisfied:[Cr]×[C]≦4;  (1)[Al]/[N]: 1 to 30; and  (2)[Mo]+0.5×[W]: 0.5 to 3.00%,  (3) wherein the bracket means a content (%)of an element written therein.
 2. The cold-work die steel according toclaim 1, further comprising at least one of the following (a) to (c):(a) V in a content of 0.5% or less (excluding 0%); (b) at least oneelement selected from the group consisting of Ti, Zr, Hf, Ta and Nb in atotal content of 0.5% or less (excluding 0%); and (c) Co in a content of10% or less (excluding 0%). 3-4. (canceled)
 5. A die obtained by usingthe cold-work die steel according to claim
 1. 6. A die obtained by usingthe cold-work die steel according to claim 2.